1. Field of the Invention
The present invention generally relates to a method for determining the frictional properties of materials, such as textiles, to better quantify values such as skin-feel, hand, and texture of the materials. While the invention is presented as being applied to textiles, the invention is also applicable to polymer films, paper sheets, and other surfaces.
2. Description of the Prior Art
Of the major aesthetical attributes of fabrics, the handle attribute is probably the most difficult to quantify. The handle or xe2x80x9chandxe2x80x9d of a fabric refers to the suppleness, softness, smoothness, flexibility, and thickness of a fabric, and relates to a complex mechanism involving physical, psychological, and neurological concepts, which combine to produce a perceived quality of the fabric. The most common method for evaluating the hand of a fabric is a subjective analysis accomplished by a person rubbing the fabric between a thumb and forefinger. However, this method is far from ideal, and the subjective nature of the test makes it difficult to establish common industry-wide parameters regarding the hand of fabrics.
F. T. Peirce (xe2x80x9cThe Handle of Cloth as a Measurable Quantityxe2x80x9d, J. Textile Inst, 21, T377-T416 (1930)) pioneered attempts to explain the hand of fabrics in terms of their physical properties, and stressed the importance of the surface properties of the fabric. Pierce did not attempt to study and measure the surface properties, but the characterization of the frictional properties of textile materials has occupied other textile researchers for many years. For example, J. A. Morrow (xe2x80x9cThe Frictional Properties of Cotton Materialsxe2x80x9d, J. Textile Inst., 22, T425-T440 (1931)) measured the frictional properties of fabrics at different normal loads, areas of contact and speeds of testing, and proposed an empirical relationship of the form:
F=mP+kAxe2x80x83xe2x80x83(1)
where F is the frictional force, P is the pressure, and A is the area of contact. It is immediately apparent from this equation that the commonly accepted Amontons Law for the coefficient of friction (xcexc=F/N), first set forth by Guillaume Amontons, would not fit Morrows data. E. C. Dreby (xe2x80x9cA Friction Meter for Determining the Coefficient of Kinetic Friction of Fabricsxe2x80x9d, J. Research Nat. Bur. Standard, 31, 237-246 (1943)) confirmed this conclusion, and a later investigation by H. G. Howell (xe2x80x9cInter-Fiber Frictionxe2x80x9d, J. Text Inst., 42, T521-T535 (1951)) provided further evidence that Amontons laws are not valid with respect to the frictional properties of fabrics.
Fundamental work by Howell and Mazur (xe2x80x9cAmontbns Law and Fibre Frictionxe2x80x9d, J. Text lnst., 44, T59-T69 (1953)) suggested that a more suitable relationship for the frictional properties of fabric was of the form:
F=CPnxe2x80x83xe2x80x83(2)
where n is a frictional index O less than n less than 1, F is the frictional force per unit area of contact, P is the normal force per unit area of contact, and C is a frictional constant. A thorough experimental investigation of this relationship was carried out by D. Wilson (xe2x80x9cA Study of the Fabric-on-Fabric Dynamic Frictionxe2x80x9d, J. Textile Inst., 54,143-155 (1963)) that confirmed its suitability. More recently Carr et al. (xe2x80x9cFrictional Characteristics of Apparel Fabricsxe2x80x9d, Textile Res. J., 58,129-136 (1988)) and J. O. Ajayi (xe2x80x9cFabric Smoothness, Friction and Handlexe2x80x9d, Textile Res. J., 62, 52-59 (1992)) have studied the effects of weave, fabric weight, direction of rubbing, etc. on the frictional properties of woven fabrics.
All of the foregoing authors attempted to define the hand quality or other properties of fabrics based upon friction constants, and with reference to Equation (2), the C value has become a well-known friction constant for quantifying the properties of fabrics. However, referring to earlier papers (Carr et al., 1988; Ajayi, 1992), where F and N are measured in physical pressure units (i.e., Pascals, which are Newtons/m2, hereinafter xe2x80x9cPaxe2x80x9d), and when solving Equation (2) for physical units, it is evident that n has no unit. However, the friction constant C has a unit Pa(1-n). Thus, it is evident from the foregoing discussion that the friction constant C is dependent on n. This dependency causes difficulty in comparing the C values of different fabrics. Furthermore, because n is a measure of the physical characteristics of the material, n values tend to vary from material to material. Thus, it is not logical to compare and characterize the frictional properties of two different textile materials using the C values. Accordingly, it is desirable to establish a new method and apparatus for quantifying the quality of a fabric or other material.
In the preferred form, the invention sets forth a method for determining the quality of a fabric or other material. The invention establishes a new constant value to be applied to this determination. The constant value is referred to as the Quality Energy Value (hereinafter the xe2x80x9cQE valuexe2x80x9d) of a material. The QE value takes into account the reciprocatory motion actually used when rubbing a piece of cloth with a finger, and utilizes a testing apparatus which somewhat simulates this to and fro motion. The QE value also advantageously takes into account the velocity at which the frictional properties of the fabric are measured. As will be demonstrated in more detail below, it has been found that the lower the QE value, the better the quality of the fabrics. Accordingly, using the method of the invention, a range of QE values may be established for a plurality of fabrics, and, thereby, the relative qualities of the fabrics may be compared.